Digital transmission system channel banks are known in the art. In telecommunication for example, a channel bank may be located in a small city and interfaced via high speed data lines with a central station high capacity digital switch in a large city. The digital switch communicates with a plurality of outlying channel banks. The channel bank receives serial high speed data in a given framing format from the digital switch and demultiplexes the data into a plurality of channels which are sent to respective channel units which in turn may further demultiplex each channel for interfacing with the various telephone subscribers. The channel units also multiplex the inputs from the various telephone subscribers up to the higher channel rate, which channels are multiplexed by the channel bank up to a high speed serial data stream for transmission back to the central station digital switch.
The demultiplexing and multiplexing operations in the channel bank are loop-timed in order to provide synchronous end to end operation with the digital switch. This loop-timing is accomplished by the use of elastic buffers and by looping the respective extracted clock signals from the low speed channel unit demultiplexers to the low speed channel unit multiplexers. Elastic buffers are inserted between the high speed demultiplexer and the low speed demultiplexers to afford a smoothing function for deleted stuff bits. Another set of elastic buffers are inserted between the low speed multiplexers and the high speed multiplexer to absorb the delta frequency between the higher order and lower order multiplexers by enabling stuffing. Extracted clock signals are looped from the outputs of the low speed demultiplexers to the inputs of the low speed multiplexers.
In higher order channel banks, a need has arisen for circuit and space efficiency. For example, in a modular type system using card cage racks with printed circuit cards or panels inserted therein, the extra dedicated rack space required by the elastic buffer cards, and their part cost, is objectionable.